What you need:
1. Fill the bottle just over half full with water.
2. Add a few drops of food colouring until you like the colour you see. You can add glitter too.
3. Fill the bottle almost to the top with vegetable oil. Let the mixture separate.
4. Break up two Alka-Seltzer tablets and drop them in the mixture. Watch your lava lamp bubble!
You can shine a torch behind or underneath your lava lamp to see it glow.
Oil is less dense than water so it sits on top of the water and food colouring mix. The water and oil separate into two layers. Alka-Seltzer tablets dissolve in the water and react to make bubbles of carbon dioxide gas. The gas bubbles rise up and take some of the coloured water with them to the surface of the oil When the gas in the bubbles escape through the top of the bottle, the water droplets fall back to the bottom. When all the gas has escaped, the water and oil separate into two layers again.
What you'll need:
Although there are a few different theories around about how this experiment works, the most favoured reason is because of the combination of carbon dioxide in the Diet Coke and the little dimples found on Mentos candy pieces.
The thing that makes soda drinks bubbly is the carbon dioxide that is pumped in when they bottle the drink at the factory. It doesn't get released from the liquid until you pour it into a glass and drink it, some also gets released when you open the lid (more if you shake it up beforehand). This means that there is a whole lot of carbon dioxide gas just waiting to escape the liquid in the form of bubbles.
Dropping something into the Coke speeds up this process by both breaking the surface tension of the liquid and also allowing bubbles to form on the surface area of the Mentos. Mentos candy pieces are covered in tiny dimples (a bit like a golf ball), which dramatically increases the surface area and allows a huge amount of bubbles to form.